Keeps the cache from getting too big.
If we were using Java 1.5, we could use LinkedHashMap and we would not need this thread
to clean out the cache.
The SortedSet sortedFilterItems is used only to sort the items from the cache,
so when it's time to clean up we have the TreeSet sort the FilterItems by
timestamp.
Removes 1.5 * the numbers of items to make the cache smaller.
For example:
If cache clean size is 10, and the cache is at 15, we would remove (15 - 10) * 1.5 = 7.5 round up to 8.
This way we clean the cache a bit more, and avoid having the cache cleaner having to do it frequently.